What is a Fungus (2024)

Fungal Diversity (Continued)

Summary of Fungal Attributes

From our discussion on how fungi are able to carry outtheir various activities, the following attributes can be used to characterizethose organisms that we classify as fungi:
1. They are heterotrophs. That is, they cannot manufacture their own food from simple compounds as plants are able to do. So they are dependent on other organisms to produce their foods, e.g., sugars, starches, proteins, fats, etc. Fungi can be further divided into saprobes, parasites, symbionts, facultative parasites and facultative saprobes.
2. The food gathering part of a fungus is made up of either filamentous, hollow, branched tubes called mycelium or are single cells called yeasts.
3. Structures called spores reproduce the fungus in the form of mycelium or yeast cells.
4. They have cell walls. This is a characteristic generally attributed to plants, but unlike plants, most fungal cell walls are composed of chitin, a carbohydrate, and is the same material which makes up the exoskeletons of insects. Plant cell walls are composed of cellulose.
5. Fungi are eukaryotes as are most other organisms with which you are familiar. However, bacteria differ from fungi in that they are prokaryotes.
6. Absorption: The process by which fungi "eat". This differs from the way in which we eat in that a fungus will digest its food before eating it. Bacteria are the only other group of organisms that eat in this fashion.

_{So What do the Set ofCharacteristics mean?}

We have gone over several workingdefinitions for "fungi" that have been accepted in mycology over thelast 50 years. Each definition, at the time that they were used was thought toinclude closely related organisms, i.e. they were all derived from a singlecommon ancestor. However, as time passed, and we gathered more information aboutthese organisms, it became apparent that this notion was not correct. So, weredefined "fungi" to reflect these changes. The above set ofcharacteristics is the most recent definition as to what we believe constitute afungus. However, in mycology, we continue to study those organisms that havebeen demonstrated to not being closely related to fungi. Thus, in mycology wehave two definition for fungi. One in which only those organisms that have theabove characteristics and are closely related to one another are recognized asbelonging to the fungus kingdom. The second includes all the fungi that havebeen recognized as fungi since Alexopoulous' definition in 1952, i.e. includesslime molds and other organisms that have been excluded from the fungus kingdom.More will be said on this subject in our lecture on "How fungi get theirname and how they are classified".

_{Post Scripts}

Sexual vs. Asexual Reproduction

Sexual reproduction is a subject that can probably be bestunderstood if we discuss it in human terms rather than using plants or fungi.You are all aware that this type of reproduction must involve two parents, andthat the children from two parents will inherit characteristics from eachparent. For example, all of you have features that can be recognized as beingmaternally or paternally inherited. This will also be true for any siblings thatyou may have. However, you and your siblings are genetically unique inappearance and personality because the process of sexual reproduction is suchthat no two individuals will be exactly alike unless they are identical twins.

Asexualreproduction requires only a single parents and the "children"produced would be genetically identical to the parent. Genetically, identicalindividuals are said to be clones. Asexual reproduction is currently moreeasily understood due to the extensive coverage of cloning in the news media ofDolly, the cloned sheep and the mice that have been cloned right here on theUniversity of Hawai‘i, Manoa campus. Although some animals, naturally, havethis type of reproduction, there are far more examples of asexual reproductionin plants. Asexual reproduction occurs when a part of an individualregenerates itself into another individual. Since this new individual wasoriginally part of the parent, the two are genetically identical. Manyagricultural plants are reproduced asexually because if you have a plant withall the qualities that you want, growing clones of this individual will ensurethat everything you are growing will also have these qualities. Examples of suchplants are illustrated in Figures. 20-22. In potatoes, each "eye" can beused to produce a new potato plant, the leafy tops of the pineapple and carrot can be grown to produce other plants.


Figures 8a, b and c. From Left to right, potatoes, pineapple, carrots and onion. These are example of plant structures that can be utilized in asexually reproducing the plant.

Both sexual and asexual reproduction hasits advantages and disadvantages. For example, if a fungus is growing in anideal environment that it is well suited for, it would be advantageous toreproduce asexually since the environment would also be ideal for clones of thatindividuals. However, if the environment should suddenly become unfavorable andlead to the death of that fungus, having an entire population of genetically,identical individuals will be a disadvantage since all of theindividuals will be equally likely to die. In this situation, sexualreproduction will be advantageous since the individuals produced will all begenetically different. Being different, there may be one to several individualsthat may be more suited for the new environment.

AFew Misconceptions About Fungi

Temperature:Fungi grow best in warm temperatures. Some species of fungi do grow better atwarm temperatures (70-90°F), but there are some that thrive in very hightemperatures of 130-150°F and some that will thrive in very low temperaturesbelow 32°F (below freezing). That is why when you refrigerate meat, it needs tobe in a freezer at -20°F. Also, how many of you have found food in yourrefrigerator that has been contaminated with fungi?

Water:Fungi need lots of water to grow. For most fungi this is true. This is why fungiare more of a problem in the tropics than in temperate areas of the world.Personal property that is normally safe from fungi, such as clothing and shoes,can be damaged in the tropics. However, some fungi can grow in very dryconditions. Dried grains and fruits can become contaminated with fungi. Fungicontaminating grains have been a very serious health hazard. For example, whenyou go into a bar, it is not the beer that will kill you, but the free peanuts.At the other extreme, there are also fungi that can live under water.

Light:Fungi can only grow in the dark. For the most part, light does not play a rolein how well fungi grow. There are some conditions where light is necessary forreproduction.

Mycelial Growth

Until the 1990's fairy rings were thoughtto be the largest examples of mycelial growth (actually rhizomorphsin this case) in fungi. In April 2, 1992 the war of the humongous fungusbegan. Smith, et al. (1992) published an article of what at that time wasconsidered to be the world's largest organisms. This article reported that whatgenetically was determined to be an individual of Armillariabulbosa, a species of mushroom, produced rhizomorphs that coveredapproximately 37 acres. It was estimated to be at least 1500 years old andweighing in at about 100 tons. This was published as a serious journal article,but captured the imagination of the news media that dubbed it the humongousfungus and a media blitz began that lasted for months. And just when things weregoing back to normal, on May 18, 1992, a still bigger mass of rhizomorph wasdiscovered. On Mt. Adams, in Washington State, rhizomorphs of Armillariaostoyae covered 1500 acres. However, even this example does notrepresent the largest example of radial, mycelial growth. The most incrediblegrowth of mycelium is one that was found in eastern Oregon, on August 1, 2000.The mycelium was found to belong to a mushroom identified as Armillariaostoyae. Researchers determined that the mycelium of this mushroom covered2,200 acres and estimated it to be over 2400 years old.The full text of this story waswritten by Volks (2002).

Literature Cited

Smith, M., J. Bruhn, and J. Anderson.1992. The fungus Armillaria bulbosa is among the oldest and largestliving organism. Nature 356:428-431

Volk, T. 2002. The Humongous Fungus --Ten Years Later. Inoculum 53(2): 4-8. (Newsletter of the Mycological Society ofAmerica)

Mycological Terms

Absorption: The means bywhich fungi obtain their food. Process begins with the release of digestiveenzymes, from the fungus, through their cell walls to digest the food that isaround them. The digested food is then "absorbed" through their cellwall.

Budding: A form ofasexual reproduction in which an outgrowth (="bud") developing on aparent yeast cell detaches to produce a new individual.